1. Technical Field of the Invention
The present invention relates to a method for acquiring a cell site station in CDMA (Code Division Multiple Access) communication systems, and more particularly, to a method for allowing a mobile station to access a base station of a cell in asynchronous CDMA cellular communication systems by using a pilot channel multiplexing two separate code sequences.
2. Description of the Prior Art
In general CDMA cellular communication systems, there are a plurality of base stations, and a pilot code of a pilot channel is used for acquiring a base station in a cell. A short code or a long code is used as the pilot code.
The long code is used as the pilot code in the conventional communication system such as an IS-95 (Interim Standard) or W-CDMA (Wideband CDMA) communication system. In the conventional system, each of the base stations uses an identical code for pilot channels and obtains timing information from GPS (Global Positioning System) equipment to synchronize transmission time of the pilot channel. In this case, since the expensive equipment, GPS, is required in each base station, the cost of the communication system can be quite high. Also, where the base station is set up in an underground space, the use of the GPS equipment has many technical difficulties.
In the conventional CDMA communication system, each of the base stations, that is, each pilot code of the base stations is discriminated by the time offset of the same PN (Pseudo Noise) sequence. For example, each base station of the IS-95 system, which has the PN sequence length 32768 and the chip rate 1.2288 Mcps, is identified with the time offset by a 64-chips, and each base station of the W-CDMA system, which has the PN sequence length 81920 and the chip rate 4.096 Mcps, is identified with the time offset by a 256-chips. Therefore, 512 base stations may be identified in the IS-95 system, and 320 base stations may be identified in the W-CDMA system.
As for each base station to use time offsets different from each other, all the base stations must have the exact timing information between them. In particular, if the inaccuracy of the timing information is larger than the time offset, it is not possible to identify each of the base stations. Therefore, in the IS-95 and W-CDMA systems, an exact synchronization between the base stations is obtained by the GPS. A communication system based on the GPS has many problems.
Since the mobile station has the reference code of the pilot code, of which the phase offset is 0, the mobile station may synchronize its phase with the phase of the base station which is denied to access, by obtaining the pilot code.
However, in an asynchronous CDMA system, because the mobile station can not obtain an accurate phase offset, the mobile station should search the phase offset for a full period of the codes. Therefore, it takes a long time for a mobile station to obtain synchronization.
Also, because the phase offset between the base stations is not constant, a difference between contiguous base stations can not be obtained. Therefore, when the handover is performed from the base station communicating with the mobile station to a new base station, there is a drawback that the mobile station should search a phase offset of a pilot code used in the new base station.
In the conventional CDMA system using short codes, each base station uses two different codes as the pilot codes. That is, to effectively use the pilot codes, pilot codes are divided to several subgroups, and a representative code representing its subgroup is added to each of the subgroups. The subgroup consists of the representative code and the constitution codes. The subgroup is used as a cluster pilot, the representative code of the subgroup as the cluster pilot code, and the constitution code as the cell pilot code, respectively. The base station provides a cluster pilot channel transferring the cluster code and a separate cell pilot channel transferring the cell code.
In other words, some cells may form one cluster. All cells associated with the same cluster use the same pilot channel, and each cell in the cluster is identified by a cell pilot code. When the mobile station sets up a call, the mobile station accesses the cluster pilot channel before it accesses the cell pilot channel.
In the above system using short codes, it takes a lot of time to access the base station, because the mobile station has no information of the base station. First, the mobile station should obtain synchronization of the cluster pilot channel. Then the mobile station receives the information of the cell codes corresponding cluster from the cluster synchronization channel. The mobile station obtains synchronization with the cell pilot channel using the received cell codes, thereby acquiring the base station.
In this case, since the mobile station accesses the cell pilot codes after searching the cluster pilot codes, the number of sequences the mobile station searches may be fairly decreased.
For example, we assume that there are 512 base stations and 512 pilot codes are divided to 16 cluster pilot codes, each of which has 32 cell pilot codes. Accordingly, the mobile station searches for 16 codes, finds its own cluster, and searches for 32 cell pilot codes. As a result, the mobile station searches for only 48 codes instead of 512 codes. This method may be used for the asynchronous CDMA systems as well as the synchronous CDMA system.
However, there still remain other problems in the method using two pilot channels for acquiring the base station. One of them is an increment of interferences between two pilot channels in the acquisition of an initial synchronization.